The research proposed in this application is designed to investigate the ontogeny and persistence of T, B, and NK cell function in infants and children with well-characterized (at both the cellular and molecular levels) human services combined immunodeficiency (SCID) who are or who become haploidentical stem cell chimeras. The unique study population will consist of 66 surviving SCIDs (and any subsequent chimeras) who have been given bone marrow stem cells without pre-transplant cytoreduction or post- transplant GVHD prophylaxis. The first aim is to examine the phenotypes, functions and genetic origins of T cells in SCID recipients of haploidentical T cell-depleted parental marrow stem cells at specified intervals post-transplantation. Two hypotheses will be tested initially: a) that the neonatal SCID thymus is able to mature normal stem cells to be phenotypically and functionally normal T cells more rapidly that thymi from older SCIDs and b) that the number and time of development of CD4+/CD45RA+ T cells post-transplantation will be predictive of more sustained and effective T cell function. The second aim is to assess the phenotype, functions and genetic origins of B cells in these chimeras as a function of time post-transplantation and type of SCID. Three hypotheses will be tested initially: a) that persistence of immature B cell phenotypes correlates with abnormal B cell development post- transplantation, b) that abnormal B cell function post-transplantation is related to the underlying molecular defect leading to SCID, and c) that the development of normal B cell function is due to the presence of donor B cells or to double parental T cell chimerism. The third aim is to assess the functions and genetic origins of NK cells longitudinally, testing the hypothesis that abnormal NK function post-transplantation is due to failure to signal through the gamma chain of the IL-15 receptor and that will only be seen in certain forms of SCID. NK function will be assessed in K562/51 Cr release assays, and purified NK cells will be analyzed for donor or host origin as indicated above. The studies in the first three aims will also reveal new information about T,B, and NK cell ontogeny not available in any other human experimental system. The fourth aim is to investigate the molecular bases of the 29% of these SCID chimeras for whom the cause remains unknown. The hypothesis to be tested is that the phenotypes of these patients' lymphocytes and/or their response or failure to respond to IL-2 will provide clues that will lead to the discovery of their underlying defects. These could include known or as yet undescribed defects, such as IL-7 or IL-7Ralpha mutations, or defects in signaling molecules such as LAT.